1. Field of the Invention
The invention relates to transparent and in particular glass substrates, which are provided with an antireflection coating, as well as to their production method. It also relates to the use thereof, particularly as glazings.
2. Discussion of the Background
An antireflection coating is usually formed by a stack of thin interface layers, generally an alternation of low and high refractive index layers. When deposited on a transparent substrate, such a coating has the function of reducing its light reflection, i.e. of increasing its light transmission. Thus, a substrate coated in this way is subject to an increase in its transmitted light to reflected light ratio, which improves the visibility of objects positioned behind it.
An antireflection coating can then be used in numerous applications, e.g. for protecting a panel illuminated by a light placed behind the observer, or for forming or constituting part of a shop display window, so as to make it easier to see what is in the window, even when the internal illumination is weak compared with the external illumination.
The performance characteristics of an antireflection coating can be measured or evaluated on the basis of different criteria. Clearly the fist criteria are of an optical nature. It can be considered that a "good" antireflection coating must be able to lower the light reflection of a standard clear glass substrate to a given value, e.g. 2%, or even 1% and less. It can also be important that the coating ensures that the substrate retains a satisfactory, e.g. neutral calorimetry, very close to that of the bare substrate. Other secondary criteria can be taken into account as a function of the envisaged application, particularly the chemical and/or mechanical durability of the coating, the cost of the materials used or the methods to be used for producing the same.
Patent application WO-92/04185 discloses an antireflection coating deposited on a transparent substrate and constituted by an alternation of layers having a high niobium oxide index and a low silicon oxide index. Its optical performance characteristics are interesting. It is advantageous to use niobium oxide from the industrial standpoint, because it is a material which can be deposited faster than other high index oxides of the titanium oxide type using known vacuum methods, such as reactive cathodic sputtering. However, it is found that such a stack is sensitive to any heat treatment and at high temperature its optical properties are unfavorably modified, particularly with respect to its colorimetry in reflection. This is disadvantageous if it is wished to give the particular substrate already provided with its coating mechanical or esthetic properties which can only be obtained by heat treatments at temperatures which may approach the softening temperature or point of the glass. Such treatment can, e.g., consist of bending or giving the substrate a certain curvature, an annealing for hardening it, or a tempering to prevent injury in the case of shattering.
One object of the invention is to obviate this disadvantage by developing a new type of antireflection, multilayer coating, which has good optical performance characteristics and which retains the latter, no matter whether or not the substrate then undergoes a heat treatment.